Method for oxidizing ferrochromium powders

ABSTRACT

Method for oxidizing ferrochromium powders which comprises establishing a fluidized bed of ferrochromium powder by flowing upwardly through a body thereof a stream of oxygen containing gas and maintaining said fluidized bed at a temperature of from 800* to about 1,050*C without supplying additional heat once the reaction has been started up.

United States Patent [191 Yamagishi et al.

m 3,821,032 [4 June 28, 1974 METHOD FOR OXIDIZING [58] Field of Search 75/5, 26; 1 17/100 M; FERROCHROMIUM POWDERS 148/63, 6.35 Inventors: Kazuo Yama'gishi, Tabaoka,

Toshitugu Banba, Kawasaki; Takeji [56] References Clted Nakajima, Chigasaki, all of Japan UNITED STATES PATENTS Tokyo, Japan 3,311,466 3/1967 Curlook 75/26 Filed: Feb. 11, 1972 Primary Exammer-Ralph S.- Kendall PP N05 225,411 Attorney, Agent, or Firm-Michael S. Striker RltdU.S.A r1 Dat eae pp ma ion a ABSTRACT Continuation-impart of Ser. No. 787,020, Dec. 26, I968, abandoned.

Foreign Application Priority Data "a fP y flowing p fi lf through 5 yt ereo a stream 0 oxygen containing gas an mam- Dec. 29, 1967 Japan 42 84730 taming a fluidized bed at a e p a of from 800 to about 1,050C without supplying additional US. Cl 148/6.35, 75/26],4785//6..5 heat once the reaction been Stamd p- Int. Cl .1 02317/04 13 Claims, 1 Drawing Figure Method for oxidizing ferrochromium powders which comprises establishing a fluidized bed of ferrochroa 4 1 l 1 l BACKGROUND or THE INVENTION This invention relates to a method for oxidizing ferrochromium powder and to an apparatus adapted for advantageously carrying out such oxidation. More particularly, this invention relates to the oxidation of high carbon ferrochromium powder for lowering the carbon content thereof.

The oxidation of high carbon ferrochromium for producing low carbon ferrochromium and the use in connection with such process of a vacuum decarbonization is well known. Several oxidation methods have already been proposed directed to the complete or partial oxidation of high carbon ferrochromium. However, the methods proposed for completely oxidizing highcarbon ferrochromium do not provide satisfactory results as the velocity at which the complete oxidation is achieved is too low to be economically acceptable. It has already been proposed to increase the reaction velocity by comminuting or grinding up the high carbon ferrochromium raw material to produce a product having a particle size of less than 325 Tyler mesh. In addition, however, in order to achieve complete oxidation,

it is required that there-beused a temperature amounting to at least l,400 to l,680C. Such high temperatures are difficult and costly to achieve, maintain and regulate. Even in the methods which have been pro posed for only partially oxidizing the surface of the high carbon ferrochromium, the reaction rates are so low that these methods can only be utilized at great cost. I

All of the known methods whether for the complete or partial oxidation require that a large amount of heat be provided. The foregoing contributes to the costliness of the methods and further gives rise to great difficulties in temperature regulation. Thus, it is known that in the known methods, an uneven temperature formation occurs in the high carbon ferrochromium layers being treated so that sintering and fusing appear in spots resulting in an uneven high oxidation and poor quality product.

It is an object of the present invention to provide a process for reducing the carbon content of high carbon ferrochromium at a satisfactory reaction velocity.

Another object is to provide a process for reducingv the carbon content of high carbon ferrochromium in a simplified and economical manner.

Still another object of the invention is to provide a method for oxidizing high carbon ferrochromium powder without materially contaminating the oxidized product with foreign materials.

Yet another object of the invention is to provide a novel apparatus useful in the production of oxidized high carbon ferrochromium powders.

Other objects, features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the drawing which diagrammatically represents an illustrative apparatus embodying the principles of the invention.

The foregoing and other objects of the invention as may appear hereinafter are achieved in a method for oxidizing ferrochromium powders which comprises establishing a fluidized bed of ferrochromium powder by bed at a temperature of from 800 to l,O50C without supplying any additional heat to the reaction once the reaction has been started up.

In accordance with the invention, it has been established that the reaction taking place in the reactor occurs mainly with respect to the chromium, the reaction being exothermic in nature. By utilizing the heat of the reaction being formed in the oxidation, the reaction can be carried out with very little external heat having to be supplied. By exactly controlling the conditions of the reaction any degree of oxidation ranging from partial to complete can be satisfactorily achieved.

The applicants have further found that if the oxidation is carried out as taught herein in a fluidized bed, low carbon ferrochromium of an extremely satisfactory nature is obtained. The oxidation in the fluidized bed is carried out so that the temperature of the high carbon ferrochromium to be oxidized and of the gas'for effecting the fluidizatio'n and oxidation is maintained within a range of 800? to l,O50C.

Thus, in accordance with the invention, by discovering the exothermic nature of the oxidation reaction and that the reaction takes place mainly with respect to the chromium which involves a comparatively weak reaction, applicants have found that it is possible tocarry out the reaction at comparatively low temperatures in the range of 800 to l,O50C. The high temperatures taught by the art, i.e., l,400-to 1.680C are not required' and accordingly the difficulties associated with such high temperatures for example, heat resistance, adiabatic properties, temperature control and/or its management are avoided making the process of the invention particularly advantageous and desirable. Further, the present invention lends itself to being carried out continuously which contributes even more to its economic desirability.

According to the method of the invention, the rate and the degree of the oxidation obtained in the ferrochromium products can be selected as desired. Thus, it is possible in accordance with the invention to choose the oxidation rate and therewith to determine the degree of oxidationrangingfromcomplete to partial oxidation enabling the recovery of products exactly as desired. v

The control of the reaction temperature can be carried out by various means including the introduction of waste gas from the reaction or another source into the furnace so that the oxygen supply is reduced, by cooling with water or other suitable liquid, by raising the quantity of auxiliary fuel supplied and the like. The time required for reaction can also be readily regulated by controlling the amount of raw material introduced into the reactor and the residence time thereof in the bed. The method of the present invention can be utilized therefore for producing oxidized ferrochromium products suitable for many and varied purposes.

The method of the invention is a very simple one and easily carried out. The method and apparatus are considerably simplified in that temperatures of 800 to l,O50C can be used rather than thevhigh temperatures of the art, i.e., 1,400 to l,680C.

The method of the invention merely requires maintaining the ferrochromium high carbon containing of gas containing free oxygen. The use of the fluidized bed technique facilitates in maintaining a uniform teming taking place due to overheating. The temperature,

as has already been noted, of the bed is easily controlled by regulating the amount of inert gas, water, auxiliary fuel, etc., present in the reactor.

It is only necessary that the air or other oxidizing gas is flowed upwardly through the bed of ferrochromium present in the reactor and that the heating conditions be adjusted to produce the desired starting temperature in the fluidized bed. In accordance with the invention, the oxidizing reaction is carried out at a temperature of from 800 to 1,050C such temperature being regulated as above set out. The oxidation reaction is then allowed to proceed without supplying additional heat for a sufficient period of time to lower the carbon content to the preselected amount.

In the preferred practice of the invention, finely ground or comminuted high carbon ferrochromium powders having a particle size of approximately less than 0.150 mm are employed although ferrochromium powders as coarse as 0.300 mm can be satisfactorily processed.

The oxidation gas can be pure oxygen, air or an oxygen enriched gas. The air oxygen or oxygen gas enriched air is blown into the bottom of the reactor at a rate which will maintain the ferrochromium powder in a fluidized condition. The fluidized gas velocities which must be used to maintain fluidization vary with the size of the powder being fed to the bed. Advantageously velocities of between 1 l and 40 cm/sec. are used,

The method of the invention provides for batchwise or continuous production. In connection with the latter, the method may be automatically controlled.

Experimental studies carried out by the inventors have established that if the temperatures of the ferrochromium powder and gas in the fluidized bed is allowed to exceed 1,050C, control of temperature becomes difficult and the reaction velocity is thereby increased resulting in even a further increase in temperature. Sintering and fusing and adverse product formation result. lfthe temperature in the fluidized bed is allowed to drop to below 800C the desired oxidation does not take place smoothly and the rate at which the oxidation does occur is economically unacceptable.

When, however. the oxidation is carried out in accordance with the invention within the range of 800 to l,050C, the oxidation is easily and successfully carried out by self-combustion in the thermally homogeneous fluidized bed. The temperature is easily and readily controlled and the desired degree and velocity of oxidation obtained without any sintering or fusing taking place. In carrying out the process of the invention, the initial charge used for starting up the reaction constitutes ferrochromium which has been partially oxidized, i.e., up to 30 percent whereby the occurrence of sintering as when the conventional raw starting materials are used is avoided. Thereafter, the conventional raw ferrochromium can be supplied, that is after the temperature in the fluidized bed has reached approximately 800C. As ferrochromium powders tend to agglomerate and sinter at the lower temperatures of 600 to 800C in order to prevent such sintering, the apparent space rate at which the initial charge is introduced, is maintained at at least above ll cm/sec. so as to insure a completely fluidized state. Preferably, while the temperature is reaching the desired value, the space rate of the solids in the fluidized zone is evenly maintained at about 15 cm/sec.

When the operating temperature of 800 to 1,050C has been reached, the space rate is maintained as constant as possible at for example 15 to 20 cm/sec. In this way, it is assured that no sintering of the ferrochromium powder occurs.

It is understood that the results obtained in the reaction depend upon the screen size of the ferrochromium powder. In accordance with the invention the screen size of the ferrochromium powder should be within the range of ASTM sieve to 300. When the screen size exceeds sieve number 100 the particles are only with considerable difficulty uniformly oxidized completely, i.e., to the center of the particles. Also when the screen size is less than sieve number 300 the particles tend to undergo fusion and sintering. Accordingly, it has been established that the best results are obtained using the range as above set out.

The process of the invention is particularly suitable for preparing high carbon ferrochromium having the properties specified by ASTM designation A-lOl. It is of course understood that the process of the invention can also be used to advantage for preparing ferrochromium having a carbon content of about 10 percent which content falls above the range specified by said ASTM standard.

The invention will be described more particularly hereinafter with reference to the drawing which shows one embodiment of an apparatus adapted for carrying out the process of the invention. 7

The raw ferrochromium is first finely divided and weighed. Thereafter, the ferrochromium is charged into hopper 8 from which it is supplied to the reactor 1 first passing through feeder 9. In the upper portion of the reactor 1, there is formed a comparatively dilute fluidized zone 16. The reactor 1 is provided with a windbox 14 in the lower portion thereof. Air and inert gas such as the waste gas of the reaction of nitrogen are introduced into the reactor through conduit 23 which is provided with a control valve 19. Recycled gas derived from the reaction is delivered via conduit 28 provided with control valves 20, 29 into the fluidized bed 15 which is formed between the fluidized bed 16 and the windbox 14. There is provided in said apparatus a blower 5 for air, a flow meter 40, a blower 6 for inert gas and a flow meter 41. Air for carrying out the oxidation and also as required for maintaining the heating is supplied under pressure to the windbox l4 and flowed upwardly through a tuyere 35 from which it is distributed into said fluidized bed 15. The air for supplying oxygen for carrying out the oxidation reaction and for maintaining the fluidized condition is introduced at a rate so that the calculated amount of oxygen and air are present in the furnace. The introduction of the air and oxygen serves to suspend the solid powder in a dense fluidized bed resembling a boiling liquid in said fluidized zone 15 between the tuyere 35 and the outlet 11. In this connection, an air feed rate of i5 to 40 cm/sec. is in general acceptable for maintaining the space rate required for fluidizing the solids. An outlet from the reactor is arranged at 11 and is regulated by providing at the aperture portion means such as a variable dam whereby the height of the fluidized zone 15 and the volume of that bed can be changed. A start-up burner 31 is provided in the upper portion of the fluidized zone 15 of the reactor 1. The burner is shut off after the reactor temperature has reached a temperature of 800-l ,500C i.e., a temperature at which self combustion takes place. After the desired temperature has been reached and the burner shut off, the reactor temperature is maintained substantially constant by selfcombustion i.e., the exothermic reaction taking place. An auxiliary fuel gun 17 is provided in the lower part of the reactor 1 and is used for changing the level of the temperature whenever the same is necessary. The auxiliary fuel gun is rarely used in continuous operation. Liquid or gaseous fuel is supplied to the fluidized bed 15 via fuel pipe 22, control valve 18 and auxiliary fuel gun 17. The combustion temperature is controlled by sensing the temperature in the fluidized bed 15 by means of a thermoelectric thermometer 32 provided in the reactor 1 or by measuring the temperature in the less dense fluidized zone at the top of the reactor by means of another thermoelectric thermometer 33. Where required, the temperature is controlled by cooling as by supplying inert gases such as waste gas of the reaction or nitrogen. The introduction of such gases serve to reduce the amount of oxygen available. Additionally, the temperature can be controlled by introducing moisture such as water or another liquid into the inert gas being fed into the reactor. It also is possible to combine the introduction of the inert gas and introduction of water. The foregoing is regulated by means of control valve 3. Alternately, it is possible to control the temperature in the reactor by controlling the amount of raw feed thereto or by controlling the residence time thereof in the reactor. The fluidized state is maintained and regulated by pressure sensing means provided at various points in the furnace using therefore manometers 36 to 38. The oxidized ferrochromium is discharged through lower outlet 11 into a cooler 47. After cooling, the ferrochromium is taken off by conveyor 13. A part of the oxidized ferrochromium is taken off with the waste gas and is discharged from the top of the furnace via a conduit 24. The ferrochromium entrained in the waste gas is collected by the cyclone 2 provided in conduit 24 and is transferred from cooler 12 at the lower portion through the damper 12 to the conveyer 13 or through the feeder to be recirculated. 1f the oxidized ferrochromium feed to the cyclone is so fine that it cannot be collected thereby, then the gas containing the same is introduced into cooler 3 via conduit 25 and passes from there through conduit 26 to the filter 4 and is separated therein. The waste gas thusly discharged is at least in part recirculated via control valve 20 of recycle line 28 from blower 6 to the windbox 14. During this time, the waste gas is suitably moistened by the humidifier 45. The waste gas not needed is discharged via drafts 42, 43 by means of fan 7. A thermoelectric thermometer 34 and manometer 39 are also provided in conduit 27. The oxidation rate of the ferrochromium powder is regulated by control of the temperature at the point where self-combustion has been reached.

Throughout the process of the invention, the temperature in the fluidized bed is maintained at 800 to l,050C and preferably at 900 to 970C plus or minus C and preferably plus or minus 10C. The process of the invention can be varied widely from a ratio of more than 1.0 for the mo] ratio of O/C of oxidized ferrochromium to that required for the theoretical oxidation of percent.

The process of the invention can be distinguished from the conventionally used procedures for oxidation of high carbon ferrochromium powders by the follow ing characteristics:

1. The particle size of the starting ferrochromium powder falls within the ASTM sieve range number 100 to 300.

2. The heating supplied from an external source is carried out until the temperature within the range of 800 to l,050C has been reached and thereafter the reaction is maintained by the heat of self combustion of the ferrochromium, no heat from any external source being required.

3. The degree of oxidation is required to be kept within the range of 1.10 to 1.20 O/C by mol.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative Examples are given. 1

Example 1 The chemical composition of a finely divided high carbon ferrochromium to be treated in accordance with the invention and having a particle size of less than 70.149 mm is set out in the following Table:

Table 1 Unit: "/1

Cr 61.2 C 7.6 Si 0.8 P 0.029 S r 0.030 Fe 30 .341

Total 100.000

The above-described high carbon ferrochromium is supplied at kg/hr; into the fluidized reactor 1, the

inside diameter of which amounts to 1.1 mm.'The ferrochromium used initially is partially oxidized up to 30 percent. There is supplied for fluidizing and oxidizing the ferrochromium 9.5 Nm"/min. of air which following start-up is reducedto 2.5 Nm /min. at 800C. The reaction proceeds smoothly without any sintering taking place. The temperature existing in the reactor amounts Table 11 Unit: 7!

Cr 53.4 C 6.0 Si 0.7 P 0.027 8 0.023 O 9.7 N Trace Fe 30 .IS

lotal 100.000

The above values are calculated mol ratios of O/C 1.21. In this connection, the chemical composition of the ferrochromium powder collected at both points was substantially the same.

The diameter of the oxidized ferrochromium particles was less than 0.149 mm (100 percent) indicating that no fusion or sintering has taken place. The thusly obtained oxidized ferrochromium (obtained with an oxidation ratio of 1.21) was briquetted and then charged into a vacuum deoxidizing furnace and there deoxidized at a temperature of from 1,200 to l,400C and a pressure of 3-0.] mmHg. The chemical composition of the products thereby obtained are set out in Table 3. The values correspond to .118 (Japanese Industrial Standards) G 2303 for low carbon ferrochromium.

The same high carbon ferrochromium as used in Example was fed into the fluidized bed reactor at a rate of 100 kg/hr. and oxidized. The reaction was selfmaintaining at the reaction temperature of 970 plus or minus C with the same operating conditions as set out in Example 1. The amount of air supply for effecting fluidization and oxidation amounted to 3.3 Nm lmin. on the average. The reaction was carried out smoothly without any sintering or fusing taking place. The amount of oxidized ferrochromium recovered at the lower outlet and in the bag filler amounted to about 105 kg/hr. and kg/hr. respectively. The composition of the collected powder is set out in Table 4.

Table IV Unit: '71

Cr 47.5 C 1.3 Si 0.6 P 0.022 S 0.020 O 27.2 N Trace Others 23 .358

Total 100.000

In view of the above set out chemical values, the-oxidation rate was equivalent to about 90 percent of the theoretical.

Example 3 metallic components present and is based on maintaining the surplus oxygen at a value dependent on-the lowest content of carbon acceptable in the product following decarbonization. Therefore the oxidation rate has to be controlled within the range of 1.1 to 1.2 O/C by mol. According to experiments which have been carried out it has been established that when the mol ratio falls below 1.1 the reduction rate and velocity thereof decrease and when the mol ratio amounts to above 1.2 the non-metallic inclusions in the final product increase. The variations corresponding to the mol ratio for a ferrochromium powder having the following composition and partially oxidized in accordance with the procedure of the invention is as follows:

Composition of Starting Ferrochromium Powder Cr C P Fe remainder Table V mol ratio the decrease in the amount of Cr and Fe does not represent a loss but is based on the combination of these elements with oxygen. I

It can be seen from the above that when the mol ratio exceeds 1.2 the content of oxygen increases rapidly. The effect of the increase in the oxygen content is also evident in that when the mo] ratio is in the range of 1.1 to 1.2 the carbon in the final product after vacuum decarbonization amounts to from 0.006 to 0.009 percent and the oxygen to 0.7 to 0.9 percent and as a result the non-metallic inclusions are very small. On the other hand when the mol ratio is 1.00, the carbon content amounts to 0.03 percent which is too high and when the mol ratio is 1.40 the carbon content is in the range of 0.002 to 0.003 percent and as a result the nonmetallic inclusions increase to 4.7 percent. Thus, in accordance with the invention the mol ratio may be very easily controlled as the oxidation reaction as carried out therein is based on the self combustion of the ferrochromium without use of any heat from an external source.

Thus in accordance with the process as herein disclosed, the oxidation exothermic reaction for treating high carbon ferrochromium may be controlled simply and easily. This results from the finding that it is the self combustion of the ferrochromium alone which is critical. This finding is utilized in accordance with the invention by excluding any heat supply from any external source as soon as the starting powder reaches a temperature of from about 800 to about 1,050C. This has the effect that any difficulty in controlling the oxidation reaction which then follows on the basis of an accelerated and uncontrollable reaction resulting from the combination of heat supplied from an external source and heat generated in the self combustion of the powder is entirely eliminated. As a result no sintering or fusing of the powder occurs and the required oxidizing rate for the powder is realized easily and without difficulty.

The foregoing Examples show that the present invention provides for completely oxidizing ferrochromium. They further establish that vacuum decarbonization can be successfully carried out with such materials.

We claim:

1. Method for oxidizing ferrochromium powders which comprises establishing and maintaining a fluidized bed of ferrochromium powder having a sieve size of less than 300, blowing upwardly through a body of such powder an oxygen containing gas at a rate of 11 to 40 cm/sec., heating said bed to a temperature of from 800C to 1,050C so as to start the oxidation of the powder, terminating the heating step when the oxidation is started, and continuing to blow the oxygen containing gas at said rate so that the oxidation degree is restricted to lie within 1.10 to 1.20 O/C by mole.

2. Method according to claim 1, wherein said ferrochromium powder is a high carbon ferrochromium powder. a

3. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by introducing an inert gas into said bed.

4. Method according to claim 1 which comprises 10 maintaining the temperatureat a predetermined value by introducing water into said bed.

5. Method according to claim 1 which comprises using a partially oxidized ferrochromium powder initially.

6. Method according to claim 1 which comprises maintaining the temperature at the desired value by regulating the dwell time of said ferrochromium in said bed.

7. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by regulating the amount of said ferrochromium powder introduced and withdrawn from said bed.

8. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by controlling the amount of auxiliary fuel supplied to said bed.

9. Method according to claim 1 wherein said ferrochromium powder has a sieve size of less than 100.

10. Method according to claiml wherein said temperature is controlled by introduction of steam or waste gas.

11. Method according to claim 1 wherein perature is maintained at 850C "to 890C.

12. Method according to claim 1 wherein said temperature is maintained at 950C to 990C.

13. Method according to claim 1 wherein said ferrochromium powder is introduced in the said fluidized bed in a partially oxidized condition.

said tem- 

2. Method according to claim 1, wherein said ferrochromium powder is a high carbon ferrochromium powder.
 3. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by introducing an inert gas into said bed.
 4. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by introducing water into said bed.
 5. Method according to claim 1 which comprises using a partially oxidized ferrochromium powder initially.
 6. Method according to claim 1 which comprises maintaining the temperature at the desired value by regulating the dwell time of said ferrochromium in said bed.
 7. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by regulating the amount of said ferrochromium powder introduced and withdrawn from said bed.
 8. Method according to claim 1 which comprises maintaining the temperature at a predetermined value by controlling the amount of auxiliary fuel supplied to said bed.
 9. Method according to claim 1 wherein said ferrochromium powder has a sieve size of less than
 100. 10. Method according to claim 1 wherein said temperature is controlled by introduction of steam or waste gas.
 11. Method according to claim 1 wherein said temperature is maintained at 850*C to 890*C.
 12. Method according to claim 1 wherein said temperature is maintained at 950*C to 990*C.
 13. Method according to claim 1 wherein said ferrochromium powder is introduced in the said fluidized bed in a partially oxidized condition. 